Semiconductor memory devices that are used for storing data can generally be divided into two classes: volatile memory devices and non-volatile memory devices. Volatile memory devices lose data stored therein when the power supply is interrupted. In contrast, non-volatile memory devices retain the stored data even when the power supply is interrupted. Therefore, nonvolatile memory devices, such as flash memory devices, are widely used in applications where power may be interrupted. For example, power may not be available. Power may occasionally be interrupted or a lower power consumption may be dictated, e.g., in a mobile phone system, a memory card for storing music and/or movie data. With increasing process capability and miniaturization, there is an increased demand for memory cells of a smaller size, even in the flash memory device.